(1) Field of the Invention
The present invention relates to an electron beam exposure apparatus employed for electron beam lithography. Particularly, it relates to a magnetic object lens for the electron beam apparatus, which is improved to reduce eddy current induced in a continuously moving stage carrying an object to be processed and to enable the normal landing o the electron beam on the object. (2) Description of the Related Art
The use of an electron beam exposure system is well-known as a method for forming minute patterns on large scale integrated circuit semiconductor devices (LSIs). As the integration density and complexity of the LSI increases, conventional optical lithographic techniques encounter a limit to the production of extremely dense LSI circuit patterns. As a result, another form of pattern-making technique with higher accuracy, such as electron beam lithography, is required. The primary advantage of electron beam lithography is its high resolution capability. Problems due to diffraction effects which are inherent to optical lithography, are resolved by electron beam lithography because the equivalent wavelength of electrons in the 10 to 20 kilovolt energy range is less than 1 .ANG. which is substantially smaller than that of an ultraviolet ray. In addition, the pattern-making is precisely controlled by a computer and is performed in short fabrication step flow, which leads to automation of a high volume production system of intricate LSI semiconductor devices. Through automation, high throughput, stable and accurate pattern-making with high yield are achieved.
Electron beam exposure systems include both electron beam projection lithography and scanning electron beam lithography. In scanning electron beam lithography, the pattern is written with a fine electron beam which is controlled by a computer to turn the beam ON and OFF and to control its deflection. The scanning electron beam system is composed of: a beam-forming system in which electrons emitted from an electron source are formed into a fine beam having a round or rectangular cross-section; a beam deflection system in which the beam is deflected in a raster or vector scanning manner, and projected onto an object such as a wafer or a mask plate (hereinafter, for convenience the medium is referred to as a wafer); and a pattern-generation and control system for controlling the scanning of the electron beam and the movement of the medium to form a required pattern on the wafer.
Generally, the scanning distance of the electron beam is limited to be within a short length, such as 2 mm, to avoid undesirable aberration. Accordingly, the entire surface of the wafer is divided into small projection sub-areas to make circuit patterns thereon. The sub-areas are individually exposed to the electron beam in timed sequence. Naturally, the wafer must be moved in synchronization with the scanning of the associated electron beam. The wafer is usually mounted on a horizontal stage which is precisely movable in X and Y directions.
There are two primary ways to move the wafer. The first is a "step and repeat" technique which has been widely used, wherein the individual patterns are formed by deflecting the electron beam over a square field (sub-area) specified on the wafer. After the completion of the pattern-making on this square field, the wafer is stepped to a new location, the electron beam is registered to a sample on the wafer, and then pattern exposure for the next square field is performed.
The second technique for moving the wafer is a "continuously moving stage" technique, which is adopted, for example, in the EBES (electron beam exposure system) developed by the Bell Telephone Laboratories, wherein the electron beam is raster scanned in one direction, and the stage is moved continuously in another direction, usually perpendicular to the scanning direction. Details of this technique are reported in IEEE Transaction on Electron Devices, Vol. RD-22, No. 7, pp. 383-392, July 1975, EBES, "A Practical Electron Lithographic System, by D. R. Heriott et al. The primary advantage of the continuously moving stage technique is that pattern writing and the movement of the wafer can occur at the same time in most cases, and fabrication time can be reduced.
In the "step and repeat" technique, the stepping period of the electron beam to a new location results in wasted time, because the move and stop motion of the stage causes a mechanical vibration of the mechanism associated with the stage due to its inertia. According to the inventors'experience, after the completion of the preceding step movement of the stage, the next patterning operation should be held for 0.3 seconds until the mechanical vibration is favorably reduced to a small amount. Assuming that the wafer is 4 inches in diameter and the square field is 2 mm square in size, then the above-described wasted time totals 520 seconds per wafer. This wasted time occupies approximately 70% of the entire time for patterning the wafer. In view of the throughput of the wafer which is patterned by a scanning electron beam system, therefore, the continuously moving stage system is more favorable for high-volume production of LSIs.
There is another problem in electron beam scanning systems which is caused by an eddy current induced in the moving stage which is subject to a strong magnetic field leaking from the magnetic object lens adjacent to the stage. The problem will be described later in more detail. In relation to this problem, it should be noted that a conventional magnetic object lens always has a circular bore opened in its lower pole piece which faces a wafer to be processed. The bore is indispensable to allow the passage of the scanning electron beam in X and Y directions. The magnetic lens system of electron beam scanning systems has been designed with a rotational axial symmetrical structure in order to eliminate aberration of the magnetic lenses to the extent possible. This appears to be the reason that the circular bore in the magnetic object lens has been adopted in prior art scanning electron beam apparatus.
In general, the primary problem with electron beam scanning systems is aberration of their magnetic lens system, particularly, that of the magnetic object lens. It is desirable that the electron beam always be projected along the axis of the magnetic object lens in order to overcome the aberration problem. Furthermore, it is desirable that the electron beam be projected onto the surface of the wafer perpendicularly, because if the electron beam is incident on the surface obliquely, distortion or discrepancy of a depicted pattern is frequently caused due to unevenness or roughness of the surface of the wafer. This desired projection of an electron beam on the wafer is referred to as a "normal-landing" on the wafer.
To meet the above-described requirements, an improved system of electron beam formation and deflection has been developed and employed in the EL3 system developed by the International Business Machine Corp. (IBM). The magnetic object lens of this system has a variable axis lens (VAL). The technique is disclosed by IBM U.S. Pat. No. 4,376,249, issued Mar. 8, 1983. The VAL is a magnetic lens with an in-lens deflection concept having two deflection coils and two compensating (or lens axis shifting) coils, with all of the coils being provided inside a pole piece of the magnetic object lens. These coils are usually electrically connected in series and enabled simultaneously. The axis of the VAL is shifted in parallel with the original lens axis by enabling the compensating coils to operate in a pair upon the deflection of the beam, so that the deflected electron beam runs in coincidence with the shifted lens axis. Since the electron beam always remains on the axis of the VAL, coma aberration and transverse chromatic aberration are substantially eliminated, and normal landing of the electron beam is realized. However, as described above, a wafer to be treated is placed on a stage movable in X and Y directions. When the stage is made of non-magnetic metal, magnetic flux which emanates from the lowermost compensating coil and lands on the stage by penetrating through the wafer, may cause eddy currents in the stage. The pole piece of the VAL has a circular bore at the bottom portion of the pole piece. Consequently, once the stage is moved, an eddy current is induced in the stage, causing the above-described deflection problem.
In order to eliminate the above-described lowermost compensating coil and prevent the disturbance of the external magnetic field, an improved variable axis magnetic object lens, referred to as variable axis immersion lens (VAIL), has been developed by IBM. This technique is disclosed in U.S. Pat. No. 4,544,846, issued Oct. 1, 1985. The VAIL has a lower pole piece, made of ferrite, which has "zero bore", and a wafer to be irradiated by an electron beam is placed on a stage mounted on the lower pole piece. Since the lower pole piece is made of ferromagnetic material, magnetic flux of the object lens lands on the surface of the lower pole piece. With this configuration, the wafer is immersed in the magnetic field and the electron beam lands on the surface of the wafer perpendicularly through the aid of magnetic flux normal to the lower pole piece. Further, since the lower pole piece has no bore and underlies the wafer, the wafer is surrounded by the structure of the VAIL, so that the structure acts as a magnetic "cage". As a result, the electron beam is shielded from the external magnetic field, and is free from the disturbance caused by the magnetic field. Thus, the electron beam is stable and can have a normal landing with respect to the wafer. However, with the structural configuration of the VAIL, the stage is also immersed in the strong magnetic field of the VAIL. Therefore, when the stage is moved, an eddy current is induced in the stage as described briefly above with respect to the VAL. The VAIL, therefore, is not suitable for continuously moving stage electron beam exposure lithography.
As described above, both the VAL and the VAIL are unsuitable for use with a continuously moving stage because of the eddy current induced in the stage which produces an adverse effect on the electron beam deflection system of the apparatus. The VAL and VAIL lenses are also reported in H.C. Pfeiffer et al., MICROCIRCUIT ENGINEERING ISBN: 0 12 15 345750 5, pps. 7214 81.
In summary, With the beam forming and scanning system of an electron beam exposure apparatus, LSI pattern-making is performed on a wafer which is carried on a continuously moving stage in a predetermined direction. Usually, the scanning direction of the electron beam is perpendicular to the moving direction of the stage. An adverse effect on proper scanning of the electron beam is caused by an eddy current induced in the moving stage which is subject to strong magnetic flux leaked through a circular bore opened in a lower pole piece of the magnetic object lens. The eddy current induces a undesirable magnetic field which deflects the scanning of the electron beam. Therefore, there is a need in the art for an electron beam exposure apparatus which overcomes this problem.